The present invention refers to a process for preparing a catalyst for the polymerization of alpha-olefins under low pressure using a Ziegler-Natta catalyst system, as well as to the spherical catalyst so obtained and to the process for preparing spherical polyethylene of ultra-high molecular weight in the presence of such catalyst. More specifically, the present invention refers to the process for preparing a spherical catalyst support, the characteristics of the support being such that the spherical shape as well as the high mechanical strength are preserved during drying, calcination and impregnation so as the catalyst prepared from the support as well as the ultra-high molecular weight polyethylene prepared from the catalyst system preserve the support spherical shape, which causes better flow properties as well as other morphological properties of the polyethylene.
In using Ziegler-Natta as catalysts for producing polymers, there is a continuous need for techniques which would lead to better processing, higher bulk density and use of lower amounts of antioxidant in the shelf, these aspects being linked to the morphology of the product polymer.
The concern with the control of the polymer morphology has been the object of numerous fundamental studies as well as of the practical, industrial and therefore patentable consequences which derive therefrom.
Thus, French patent FR 2,071,111, owned by Solvay, teaches that supported catalysts allow for the absolute control of the polymer morphology, the morphologies of the support and the polymer being linked. This can be stated in the case where the support has the shape of a microsphere, the polymer obtained having the shape of small spheres, as set forth in French patent 1,550,186. In FR 2,071,111, a metal halide of Groups IV, V and VI of the Periodical Table in its maximum valence state is reduced on a support by means of an organic compound such as an aluminum alkyl, the support being previously impregnated with one of the reagents which make up the catalyst, in the liquid state while introducing the impregnated support into the other reagent which is found either pure in the liquid state, either dissolved in a solvent. It is alleged that a correct kind of support for the objectives of the patent are the so-called xe2x80x9ccenospheresxe2x80x9d which are made up of porous spheres of diameter between 50 and 250 microns, each sphere being a collection of units of diameter between 0.2 to 2 microns. Thus, while the external shape of the cenosphere determines the morphology of the polymer produced with the aid of the supported catalyst, it could equally be seen that the elementary particles which constitute the cenosphere are regularly spread on the polymer. These particles can act as nucleating centers when the polymer crystallizes. There is a comment in this reference that due to the fact that the polymer formed is an increased image of the support, the cenosphere granulometry is reflected on the granulometry of the polymer beads and consequently influences the bulk density of the polymer. Normally, a high bulk density is sought which is obtained from a support of wide distribution of particle sizes, especially a bimodal distribution, maxima being found at 55 microns and 125 microns. The morphology and bulk density of the support are equally monitored by the choice of the support, which makes possible to reach slurries of high densities during polymerization while the particle size of the polymer which exits the polymerization vessel is such that it does not require granulation. As a consequence of the effect of the support, a high activity, good morphology catalyst is produced. The described catalysts are useful in the polymerization or co-polymerization of all alpha-olefins.
A. Muxc3x1oz-Escalona, in an article published in the Polymer Preprints of the American Chemical Society, Division of Polymer Chemistry, 24(1), 112-13 (1983), states that the catalyst support, more than the polymerization technique, controls the morphology of the polymer particles obtained through supported Ziegler-Natta catalysts. In another article by A. Muxc3x1oz-Escalona and A. Sierraalta, published by the Acta Cient. Venez. 34 (3-4), p. 203-8 (1983), the authors teach that in the ethylene polymerization catalyzed by Et2AlClxe2x80x94TiCl4 the Al/Ti ratio is the most important factor affecting the morphology of produced polyethylene, an increase of this ratio causing an increase in the crystallinity and the density as well as an increase in the particle size of the polymer. As the Al/Ti ratio increases the bulk density also increases while the molecular weight is reduced.
EP 252804 describes catalysts the morphology of which is preserved during polymerization. This patent teaches that ethylene is polymerized on spherical catalysts which contain transition metals, magnesium compounds as well halides up to an adequate degreee of polymerization, the catalyst being then treated with the aluminum compounds to stabilize the spherical morphology. In EP 468070 (corresponding to Japanese patent JP 221112) in the name of J. Kano et al., entitled xe2x80x9cProcess for Preparing Spherical Silica Gelxe2x80x9d, a method is described for preparing spherical silica gel wherein the amount of water present in the paste is adjusted to be of from 0.2 to 1.5 times the weight of silica hydrogel, in the process for preparing spherical silica gel during the spray drying of the paste of silica hydrogel and water. The silica hydrogel paste is obtained by reaction of the alkali metal silicate salt and mineral acid followed by humid granulation of the hydrogel silica, the pH of the silica hydrogel being in the range of from 1 to 3. Although it is alleged that the obtained spherical silica is adequate as catalyst support, no significant example of the produced silica as catalyst support is provided. In Brazilian patent PI BR 8005302, of the Applicant and hereby fully incorporated as reference, is described a process for preparing an alumina useful as catalyst support or as a catalyst from the reaction of aluminum sulfate and ammonium bicarbonate at 15-20xc2x0 C., the pH being maintained between 7.5 and 7.7 through the addition of ammonium hydroxide, to produce the precursor ammonium dawsonite, which contains of from 10 to 20 weight % of residual sulfate ions. The calcination of the ammonium dawsonite at 600-800xc2x0 C. for 4-10 hours yields an alumina of surface area 200-400 m2/g, pore volume 1.5 to 3.5 cm3/g and where 85% of the pores are greater than 100 A. In order to avoid sulfate losses, the precursor ammonium dawsonite is not washed prior to calcination.
In U.S. Pat. No. 4,983,693, corresponding to Brazilian patent 8707098, of the Applicant and herein fully incorporated by reference, a catalyst for the polymerization of alpha-olefins is described which is obtained by impregnating the alumina taught in Brazilian patent 8005302 with of from 0.8 to 1.0 weight % of titanium from titanium halide in n-hexane, activated by triisobutyl aluminum or triethylaluminum, the molar ratio of Al/Ti in the catalyst being 15/1 up to 60/1. The thus obtained polyethylene has ultra-high molecular weight and is used as an engeneering plastics in view of its outstanding mechanical properties, chiefly high impact and abrasion strength as well as high tensile strength. However, the polyethylenes obtained through such a process show a drawback as regards their morphological properties, that is, particles are irregular and of low bulk density (0.25 to 0.30 g/cm3). Additives can be added to the polymers to increase their bulk density; however, this practice increases cost as well as impurities in the finished product. The irregular morphology of the polymer particles produced according to the process of U.S. Pat. 4,983,693 necessarily causes fluidity problems which reflect directly on the polymer processing and storage. Besides, irregular polymer particles require higher antioxidant amountsxe2x80x94neatly toxicxe2x80x94which severely limits its use in the food industry.
In pressing morphologically irregular polymers, defficient flow and packing cause air bubbles in the pressed items, which then show inferior abrasion strength.
Thus, it can be seen that, in spite of the existence of numerous academic studies on the morphology of catalyst supports, the catalyst made from those supports and their influence on the produced polymer, as well as patents which suggest the use of spherical supports as being able to convey the spherical shape to the polymer (replication phenomenon), the scientific literature has not yet published nor suggested a process or a support for a Ziegler catalyst which would be easily prepared by the industry in spherical shape, and which after calcination and impregnation keeps mechanical properties intact, chiefly good wear strength, so as to convey to the polymer the replication phenomenon. The replication phenomenon causes that the support or catalyst conveys to the polymer its own morphology, the product showing then optimum characteristics as regards bulk density and molecular weight, with low requirements in anti-oxidant.
One objective of the invention is a spherical catalyst support based on ammonium dawsonite which is able, by means of the replication phenomenon, to convey its morphological properties to the catalyst as well as to the produced polymer.
Another objective is a spherical support which is prepared by the spray-drying of the ammonium dawsonite slurry, the morphological properties of which are preserved in the catalyst and fully conveyed to the produced polymer, which will then show optimum properties as concerns bulk density and flow, while keeping at a minimum the need of anti-oxidant additives.
Still another objective is a spherical, easily processable polyolefin powder, which requires low or no amounts of additive for improving bulk density and oxidation resistance.
These objectives are attained by using a well-known techniquexe2x80x94spray-dryingxe2x80x94on an active support, of high surface area, which leads to a spherical product of new features, which, through the replication phenomenon, have been conveyed to the product polymer, of excellent morphology.